Thanks to the old man’s hard work, a lot of has been covered with green trees.A. the hill

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A. the hills
B. hills
(答案→)C. the hill
D. hill
解析：解此题的关键词是谓语动词has，可见主语是单数意义的名词，可以先排除A、B，其次要注意该名词前面有a lot of来修饰，此时的a lot of不是指“许多”，而是相当于a large part of，后跟表示特定意义的名词the hill，意思是“这座山的多数地方”